where a dimensionless time T — vt/l. Constrain the distribution functions by the boundary conditions F(xi) = Fi, F(xo) = 0. The constant Fi = (2^)-1 if the region x > xi is occupied by particles and the distribution function is normalized to unit.

The second condition corresponds to the assumption that the particles coming onto the boundary x — xo are immediately removed into the backward hemisphere of the angular space. This approximation is sufficient for evaluation of the order of the isotropization time. Solution of the Eq. 2.7.14 with the boundary conditions described has the form

xo J

where An =m ln(x^xo ); the factors An and the coefficients An are determined by the initial condition. The dimensionless time of filling of the zone xo < x < x1 as it results from Eq. 2.7.15 is of the order of

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